1. Field of the Invention
The invention relates generally to the forming of electrical equipment covers, and more particularly, this invention relates to a method of forming electrical equipment covers containing non-magnetic material to resist heating induced by the presence of varying magnetic fields.
2. Description of the Prior Art
It has long been known that magnetic fields surround wires carrying electric currents and that these fields can affect the functioning and properties of nearby materials. It has also been known that the magnetic fields which surround changing electric currents, such as alternating current, are themselves changing, and that these changing magnetic fields, sometimes called time-varying, produce somewhat different effects than static fields. In particular, it has been long known that time-varying magnetic fields can produce heating in certain metals. Two phenomena are involved -- hysteresis and eddy current losses.
When a time-varying magnetic field contacts an electrical conductor, be the conductor magnetic or not, the field excites the electrons, causing them to move, thus producing an electric current. This electric current produces more or less heat, depending upon the electrical properties of the conductor carrying it. Since these induced currents tend to travel in circles, the name eddy currents has been used to describe them, but the term Foucault current is also commonly used.
The second phenomenon involved in the heating of magnetic conductors in a time-varying magnetic field is the hysteresis effect. A magnetic conductor is described in terms of a plurality of magnets. The presence of a magnetic field causes these magnets to align themselves in the direction of the field, thus causing disturbances in the interior of the conductor. When the magnetic field is constantly reversing, as is the case when it is produced by alternating current, the numerous magnets are also constantly reversing to align themselves with the field. This constant alignment process results in the retention of energy (hysteresis) and is the source of the heat perceived when a magnetic conductor is surrounded by a time-varying magnetic field.
While the existence of eddy current and hysteresis losses are known, the precise measurement of these effects is extremely difficult. For example, in the attempt to measure exactly the amount of heat produced by eddy currents, the so-called "eddy current anomaly" was discovered, in which the heat produced, for no apparent reason, often exceeded that predicted. Similarly, when both eddy currents and hysteresis are at work, it has proved difficult to determine the share of the heat produced by each, and thus difficult to determine whether the heating is due to the electrical or magnetic properties of the metal. Furthermore, it is known that the nature of the two phenomena is significantly affected by the geometrical shape of the material, by the composition of the material, and by any boundaries where dissimilar materials meet.
In any event, the heating resulting from eddy currents and hysteresis effects creates problems in several respects. For one thing, it can weaken the heated material during the period of temperature increase, as well as destroying the benefits of tempering the material. Further, the heating can cause the material to expand and distort, possibly dislocating components whose stationary position is important (e.g., current-carrying bushings). The heating can also adversely affect the electromagnetic properties of the material itself, properties such as its resistance and reluctance. In addition, the eddy currents can induce their own magnetic fields, which can cause interference in nearby electrical circuits. Still further, the heated material may be a tank containing an electrical component surrounded by an insulating fluid such as oil, which is the situation in which the preferred embodiment of this invention is utilized. The heat causes thermal decomposition of the oil with an accompanying alteration in its insulating properties, with the result that it must be replaced.
In attempting to obviate the problems of heating, prior inventors have attempted to minimize the eddy current and hysteresis losses. Thus, one solution has been to arrange the magnetic fields so that they are equal but opposite in direction so that they cancel each other. Another solution has been to tolerate the heat produced, but to direct it away from critical areas by using metals having good heat conductivity. Another has been to employ shields of materials which are themselves non-magnetic and thus resistant to hysteresis heating. Some of these metals are also relatively impermeable to magnetic fields, and this feature has been used to provide a shield which does not itself become heated and which also cuts off magnetic fields from reaching vulnerable components. It will be noted that these solutions rely on three methods of operation: they remove the magnetic fields by cancellation or shielding, or they remove the heat produced, or they use a non-magnetic material which is not heated by hysteresis.
With specific reference to covers for electrical equipment, such as circuit breakers and transformers, two basic approaches have been utilized in the past. One has been to construct the cover completely from non-magnetic material, such as a non-magnetic stainless steel. While this approach resolves the hysteresis and eddy current problems stemming from magnetic fields, such non-magnetic material is quite expensive relative to ferromagnetic structural materials that would otherwise be utilized. Accordingly, the expense of this approach renders it prohibitive for most larger electrical equipment situations.
The other approach that has been employed is to construct the casing of the lower cost ferromagnetic material (e.g., low carbon steel), and then replace portions of the ferromagnetic material with strips of non-magnetic material. This is accomplished by cutting slots in the cover which extend between openings provided in the cover for the attachment of current-carrying bushings, the strips of non-magnetic material being located in the slots. By use of this approach, the magnetic fields producing the eddy current and hysteresis effects encounter a high reluctance, which considerably reduces the heating of the cover produced by the magnetic field.
While this latter approach greatly reduces the hysteresis and eddy current losses at a relatively low cost, it involves other problems. Past methods of installing the strips of non-magnetic material have involved cutting the slots in the casing and welding the strips to the casing after it was formed. This welding of the already formed casing produced distortion thereof. Since there are close tolerances for the bushings in order to accurately align the contacts and to provide a desired seal, the distortion resulting from the welding creates problems. To overcome these problems, it has been necessary to employ adjustable bushings to correct for the misalignments resulting from distortion of the casing.